|Publication number||US7057223 B2|
|Application number||US 10/879,378|
|Publication date||Jun 6, 2006|
|Filing date||Jun 29, 2004|
|Priority date||Oct 6, 1997|
|Also published as||US6066869, US6764901, US7223678, US20020006699, US20040235243, US20060003525|
|Publication number||10879378, 879378, US 7057223 B2, US 7057223B2, US-B2-7057223, US7057223 B2, US7057223B2|
|Inventors||Wendell P. Noble, Leonard Forbes|
|Original Assignee||Micron Technology, Inc|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (100), Non-Patent Citations (99), Referenced by (10), Classifications (12), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation of U.S. application Ser. No. 09/551,027 filed Apr. 17, 2000, now issued as U.S. Pat. No. 6,764,901 on Jun. 20, 2004, which is a divisional of U.S. application Ser. No. 08/939,742 filed Oct. 6, 1997, now issued as U.S. Pat. No. 6,066,869 on May 23, 2000. These applications are incorporated herein by reference.
The present invention relates generally to the field of memory devices and, in particular, to a circuit and method for a folded bit line memory cell with a vertical transistor and a trench capacitor.
Electronic systems typically store data during operation in a memory device. In recent years, the dynamic random access memory (DRAM) has become a popular data storage device for such systems. Basically, a DRAM is an integrated circuit that stores data in binary form (e.g., “1” or “0”) in a large number of cells. The data is stored in a cell as a charge on a capacitor located within the cell. Typically, a high logic level is approximately equal to the power supply voltage and a low logic level is approximately equal to ground.
The cells of a conventional DRAM are arranged in an array so that individual cells can be addressed and accessed. The array can be thought of as rows and columns of cells. Each row includes a word line that interconnects cells on the row with a common control signal. Similarly, each column includes a bit line that is coupled to at most one cell in each row. Thus, the word and bit lines can be controlled so as to individually access each cell of the array.
A memory array is typically implemented as an integrated circuit on a semiconductor substrate in one of a number of conventional layouts. One such layout is referred to as an “folded digit line” architecture. In this architecture, sense amplifier circuits are provided at the edge of the array. The bit lines are paired in complementary pairs. Each complementary pair in the array feeds into a sense amplifier circuit. The sense amplifier circuit detects and amplifies differences in voltage on the complementary pair of bit lines as described in more detail below.
To read data out of a cell, the capacitor of a cell is accessed by selecting the word line associated with the cell. A complementary bit line that is paired with the bit line for the selected cell is equilibrated with the voltage on the bit line for the selected cell. The equilibration voltage is typically midway between the high and low logic levels. Thus, conventionally, the bit lines are equilibrated to one-half of the power supply voltage, VCC/2. When the word line is activated for the selected cell, the capacitor of the selected cell discharges the stored voltage onto the bit line, thus changing the voltage on the bit line.
The sense amplifier detects and amplifies the difference in voltage on the pair of bit lines. The sense amplifier typically includes two main components: an n-sense amplifier and a p-sense amplifier. The n-sense amplifier includes a cross-coupled pair of n-channel transistors that drive the low bit line to ground. The p-sense amplifier includes a cross-coupled pair of p-channel transistors and is used to drive the high bit line to the power supply voltage.
An input/output device for the array, typically an n-channel transistor, passes the voltage on the bit line for the selected cell to an input/output line for communication to, for example, a processor of a computer or other electronic system associated with the DRAM. In a write operation, data is passed from the input/output lines to the bit lines by the input/output device of the array for storage on the capacitor in the selected cell.
Each of the components of a memory device are conventionally formed as part of an integrated circuit on a “chip” or wafer of semiconductor material. One of the limiting factors in increasing the capacity of a memory device is the amount of surface area of chip used to form each memory cell. In the industry terminology, the surface area required for a memory cell is characterized in terms of the minimum feature size, “F,” that is obtainable by the lithography technology used to form the memory cell. Conventionally, the memory cell is laid out with a transistor that includes first and second source/drain regions separated by a body or gate region that are disposed horizontally along a surface of the chip. When isolation between adjacent transistors is considered, the surface area required for such a transistor is generally 8F2 or 6F2.
Some researchers have proposed using a vertical transistor in the memory cell in order to reduce the surface area of the chip required for the cell. Each of these proposed memory cells, although smaller in size from conventional cells, fails to provide adequate operational characteristics when compared to more conventional structures. For example, U.S. Pat. No. 4,673,962 (the '962 patent) issued to Texas Instruments on Jun. 16, 1997. The '962 patent discloses the use of a thin poly-silicon field effect transistor (FET) in a memory cell. The poly-silicon FET is formed along a sidewall of a trench which runs vertically into a substrate. At a minimum, the poly-silicon FET includes a junction between poly-silicon channel 58 and the bit line 20 as shown in FIG. 3 of the '962 patent. Unfortunately, this junction is prone to charge leakage and thus the poly-silicon FET may have inadequate operational qualities to control the charge on the storage capacitor. Other known disadvantages of such thin film poly-silicon devices may also hamper the operation of the proposed cell.
Other researchers have proposed use of a “surrounding gate transistor” in which a gate or word line completely surrounds a vertical transistor. See, e.g., Impact of a Vertical Φ-shape transistor (VΦT) Cell for 1 Gbit DRAM and Beyond, IEEE Trans. On Elec. Devices, Vol 42, No.12, December, 1995, pp. 2117–2123. Unfortunately, these devices suffer from problems with access speed due to high gate capacitance caused by the increased surface area of the gate which slows down the rise time of the word lines. Other vertical transistor cells include a contact between the pass transistor and a poly-silicon plate in the trench. Such vertical transistor cells are difficult to implement due to the contact and should produce a low yield.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for realizable memory cell that uses less surface area than conventional memory cells.
The above mentioned problems with memory cells and other problems are addressed by the present invention and which will be understood by reading and studying the following specification. A memory cell is described which includes a vertical transistor and trench capacitor.
In particular, an illustrative embodiment of the present invention includes a memory cell for a memory array with a folded bit line configuration. The memory cell includes an access transistor that is formed in a pillar of single crystal semiconductor material. The access transistor has first and second sources/drain regions and a body region that are vertically aligned. The access transistor also includes a gate that is coupled to a wordline disposed adjacent to the body region of the access transistor. A passing wordline is separated from the gate by an insulator for coupling to other memory cells adjacent to the memory cell. A trench capacitor is also included. The trench capacitor includes a first plate that is formed integral with the first source/drain region of the access transistor and a second plate that is disposed adjacent to the first plate and separated from the first plate by a gate oxide. In another embodiment, the second plate of the trench capacitor surrounds the second source/drain region. In another embodiment, an ohmic contact is included to couple the second plate to a layer of semiconductor material.
In another embodiment, a memory device is provided. The memory device includes an array of memory cells. Each memory cell includes a vertical access transistor that is formed of a single crystalline semiconductor pillar that extends outwardly from a substrate. The semiconductor pillar includes a body and first and second source/drain regions. A gate is disposed adjacent to a side of the pillar adjacent to the body region. The memory cell also includes a trench capacitor wherein a first plate of the trench capacitor is integral with the first source/drain region and a second plate of the trench capacitor is disposed adjacent to the first plate. The memory device also includes a number of bit lines that are each selectively coupled to a number of the memory cells at the second source/drain region of the access transistor. This forms columns of memory cells in a folded bit line configuration. Finally, the memory device also includes a number of wordlines. The wordlines are disposed substantially orthogonal to the bit lines in trenches between rows of the memory cells. Each trench includes two wordlines. Each wordline is coupled to gates of alternate access transistors on opposite sides of the trench. In another embodiment, the pillars extend outward from a semiconductor portion of the substrate. In another embodiment, a surface area of the memory cell is four F2, wherein F is a minimum feature size. In another embodiment, a second plate of the trench capacitor surrounds the second source/drain region of the access transistor. In another embodiment, the second plate of the trench capacitor is maintained at approximately ground potential. In another embodiment, the pillar has a sub-micron width so as to allow substantially full depletion of the body region.
In another embodiment, a memory array is provided. The memory array includes a number of memory cells forming an array with a number of rows and columns. Each memory cell includes an access transistor with body and first and second source/drain regions formed vertically, outwardly from a substrate. A gate is disposed adjacent to a side of the transistor. The memory array includes a number of first isolation trenches that separate adjacent rows of memory cells. First and second wordlines are disposed in each of the first isolation trenches. The first and second wordlines are coupled to alternate gates on opposite sides of the trench. The memory array also includes a number of second isolation trenches, each substantially orthogonal to the first isolation trenches and intraposed between the adjacent memory cell.
In another embodiment, a method of fabricating a memory array is provided. A number of access transistors were formed wherein each access transistor is formed in a pillar of semiconductor material that extends outwardly from a substrate. The access transistor includes a first source/drain region, a body region and second source/drain region that are formed vertically. The method also includes forming a trench capacitor wherein a first plate of the trench capacitor is integral with the first source/drain region of the access transistor. Further, the method includes forming a number of wordlines in a number of trenches that separates adjacent rows of access transistors. Each trench includes two wordlines with the gate of each wordline interconnecting alternate access transistors on opposite sides of the trench. Finally, the method includes a number of bit lines that interconnect second source/drain regions of selected access transistors.
In another embodiment, a method of fabricating a memory is provided. The method begins with forming a first conductivity type first source/drain region layer on a substrate. A second conductivity type body region layer is formed on the first source/drain region layer. A first conductivity type second source/drain region layer is formed on the body region layer. Additionally, a plurality of substantially parallel column isolation trenches are formed extending through the second source/drain region layer, the body region layer and the first source/drain region layer. This provides column bars between the column isolation trenches. Further, a plurality of substantially parallel row isolation trenches are formed orthogonal to the column isolation trenches and extending to substantially the same depth as the column isolation trenches. This produces an array of vertical access transistors for the memory array. The row and column isolation trenches are filled with a conductive material to a level that does not exceed the lower level of the body region so as to provide a common plate for capacitors of the memory cells of the memory array. Two conductive wordlines are formed in each row isolation trench to selectively interconnect alternate access transistors on opposite sides of the row isolation trench. Finally, bit lines are formed to selectively interconnect the second source/drain regions of the access transistors on each column.
In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.
In the following description, the terms wafer and substrate are interchangeably used to refer generally to any structure on which integrated circuits are formed, and also to such structures during various stages of integrated circuit fabrication. Both terms include doped and undoped semiconductors, epitaxial layers of a semiconductor on a supporting semiconductor or insulating material, combinations of such layers, as well as other such structures that are known in the art.
The term “horizontal” as used in this application is defined as a plane parallel to the conventional plane or surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizonal as defined above. Prepositions, such as “on”, “side” (as in “sidewall”), “higher”, “lower”, “over” and “under” are defined with respect to the conventional plane or surface being on the top surface of the wafer or substrate, regardless of the orientation of the wafer or substrate.
In the exemplary embodiment of
The vertical transistors are laid out in a substantially checker-board pattern of rows and columns on a substrate. Memory cell 112-11 includes vertical transistor 130-11. A source/drain region of transistor 130-11 is formed in a deep trench and extends to a sufficient depth to form a storage node of storage capacitor 132-11. The other terminal of storage capacitor 132-11 is part of a mesh or grid of poly-silicon that surrounds the source/drain region of transistor 130-11 and is coupled to ground potential.
The N word lines, WL-1 through WL-N, are formed in trenches that separate adjacent rows of vertical transistors 130-ij. Each trench houses two word lines, with each word line in a trench acting as a gate for alternate transistors on one side of the trench.
Bit lines BL-1 through BL-M are used to write to and read data from memory cells 112-ij in response to addressing circuitry. For example, address buffer 114 is coupled to control bit line decoder 118, which also includes sense amplifiers and input/output circuitry that is coupled to bit lines BL-1 through BL-M and complement bit lines BL-1* through BL-M* of array 110. Address buffer 114 also is coupled to control word line decoder 116. Word line decoder 116 and bit line decoder 118 selectably access memory cells 112-ij in response to address signals that are provided on address lines 120 from electronic system 101 during write and read operations.
In operation, memory 100 receives an address of a particular memory cell at address buffer 114. For example, electronic system 101 may provide address buffer 114 with the address for cell 112-11 of array 110. Address buffer 114 identifies word line WL-1 for memory cell 112-11 to word line decoder 116. Word line decoder 116 selectively activates word line WL-1 to activate access transistor 130-1 j of each memory cell 112-1 j that is connected to word line WL-1. Bit line decoder 118 selects bit line BL-1 for memory cell 112-11. For a write operation, data received by input/output circuitry is coupled to bit lines BL-1 through access transistor 130-11 to charge or discharge storage capacitor 132-11 of memory cell 112-11 to represent binary data. For a read operation, bit line BL-1 of array 110 is equilibrated with bit line BL-1*. Data stored in memory cell 112-11, as represented by the charge on its storage capacitor 132-11, is coupled to bit line BL-1 of array 110. The difference in charge in bit line BL-1 and bit line BL-1* is amplified, and a corresponding voltage level is provided to the input/output circuits.
Each memory cell is constructed in a similar manner. Thus, only memory cell 202C is described herein in detail. Memory cell 202C includes pillar 204 of single crystal semiconductor material, e.g., silicon, that is divided into second source/drain region 206, body region 208, and first source/drain region 210 to form access transistor 211. Pillar 204 extends vertically outward from substrate 201 of, for example, p− silicon. Second source/drain region 206 and first source/drain region 210 each comprise, for example, n+ silicon and body region 208 comprises p− silicon.
Word line 212 passes body region 208 of access transistor 211 in isolation trench 214. Word line 212 is separated from body region 208 of access transistor 211 by gate oxide 216 such that the portion of word line 212 adjacent to body region 208 operates as a gate for access transistor 211. Word line 212 may comprise, for example, n+ poly-silicon material that is deposited in isolation trench 214 using an edge-defined technique such that word line 212 is less than a minimum feature size, F, for the lithographic technique used to fabricate array 200. Passing word line 213 is also formed in trench 214. Cell 202C is coupled with cell 202B by bit line 218.
Memory cell 202C also includes storage capacitor 219 for storing data in the cell. A first plate of capacitor 219 for memory cell 202C is integral with first source/drain region 210 of access transistor 211. Thus, memory cell 202C may be more easily realizable when compared to conventional vertical transistors since there is no need for a contact between first source/drain region 210 and capacitor 219. Second plate 220 of capacitor 219 is common to all of the capacitors of array 200. Second plate 220 comprises a mesh or grid of n+ poly-silicon formed in deep trenches that surrounds at least a portion of first source/drain region 210 of each pillar 204A through 204D. Second plate 220 is grounded by contact with substrate 201 underneath the trenches. Second plate 220 is separated first source/drain region 210 by gate oxide 222.
With this construction for memory cell 202C, access transistor 211 is like a silicon on insulator device. Three sides of the transistor are insulated by thick oxide in the shallow trench. If the doping in pillar 204 is low and the width of the post is sub-micron, then body region 208 can act as a “fully-depleted” silicon on insulator transistor with no body or substrate to contact. This is desirable to avoid floating body effects in silicon on insulated transistors and is achievable due to the use of sub-micron dimensions in access transistor 211.
As shown in
As shown in
A thin layer of silicon dioxide (SiO2), referred to as pad oxide 308, is deposited or grown on layer 306. Pad oxide 308 has a thickness of approximately 10 nanometers. A layer of silicon nitride (Si3N4), referred to as pad nitride 310, is deposited on pad oxide 308. Pad nitride 310 has a thickness of approximately 200 nanometers.
Photo resist layer 312 is deposited outwardly from layer 310. Photo resist layer 312 is patterned with a mask to define openings 314 in layer 312 to be used in selective etching. As shown in
A thin thermal protective oxide layer 318 is grown on exposed surfaces of substrate 300 and layers 302, 304, and 306. Layer 318 is used to protect substrate 300 and layers 302, 304 and 306 during subsequent process step.
A layer of intrinsic poly-silicon 320 is deposited by chemical vapor deposition (CVD) to fill column isolation trenches 316. Layer 320 is etched by reactive ion etching (RIE) such that layer 320 is recessed below a top of layer 302. Layer 322 of silicon nitride (Si3N4) is deposited by, for example, chemical vapor deposition to fill trenches 316. Layer 322 is planarized back to a level of layer 310 using, for example, chemical mechanical polishing (CMP) or other suitable planarization technique to produce the structure shown in
As shown in
As shown in
As shown in
An optional metal contact 336 may be formed by, for example, deposition of a collimated refractory metal deposition, e.g., titanium, tungsten, or a similar refractory metal. This provides an ohmic metal contact for a capacitor plate on a surface 335 of substrate 300.
Dielectric layer 338 is deposited or grown on sidewalls of layer 302 of pillars 334A through 334D. Layer 338 acts as the dielectric for the storage capacitors of array 299 of memory cells. If contact 336 was previously deposited on a surface of substrate 300, dielectric layer 338 should be directionally etched to clear dielectric material from the bottom of row isolation trench 332.
Next, a common plate for all of the memory cells of array 299 is formed by a chemical vapor deposition of N+ poly-silicon or other appropriate refractory conductor in column isolation trenches 316 and row isolation trenches 332. In this manner, conductor mesh or grid 340 is formed so as to surround each of pillars 334A through 334D. Mesh 340 is planarized and etched back to a level approximately at the bottom of the nitride bridge formed by nitride layers 322 and 310 as shown in
As shown in
Next, a brief oxide etch is used to expose the top surface of intrinsic poly-silicon layer 358. Layer 358 is then selectively etched to remove the remaining intrinsic poly-silicon using an etchant such as KOH and alcohol, ethylene and pyrocatechol or gallic acid (as described in U.S. Pat. No. 5,106,987 issued to W. D. Pricer). Next, an oxide layer is deposited by, for example, chemical vapor deposition to fill the space vacated by layer 358 and to fill in between word line conductors 366. Additionally conventional process steps are used to add bit lines 368 so as to produce the structure shown in
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. For example, the semiconductor materials and dimensions specified in this application are given by way of example and not by way of limitation. Other appropriate material can be substituted without departing from the spirit and scope of the invention.
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|U.S. Classification||257/296, 257/301, 257/E21.652, 257/E27.095, 257/300|
|International Classification||H01L27/108, H01L29/76, H01L21/8242|
|Cooperative Classification||H01L27/10864, H01L27/10838|
|European Classification||H01L27/108M4B6B, H01L27/108F10|
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|Jan 17, 2014||REMI||Maintenance fee reminder mailed|
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